We propose to characterize histamine H1 receptors in the brain and peripheral tissues by the binding of 3H-drugs such as mepyramine and doxepin. The possibility of multiple receptors will be explored as well as regulation by guanine nucleotides, sodium and divalent cations. Histidine decarboxylase will be isolated from a variety of sources and characterized biochemically. Antisera will be raised against the purified enzyme and used to identify histamine synthesizing neurons in the brain by immunohistochemical procedures. Histamine H1 and H2 receptors, GABA and benzodiazepine receptors will be solubilized and purified. We will attempt to separate possible multiple histamine, GABA and benzodiazepine receptors and characterize their differential properties.